Semiconductor nanocrystals (quantum dots) have drawn a great deal of attention in the past decades because of their unique size-dependent property, namely “quantum confinement effect.” The quantum dots have been applied in a wide spectrum of fields, such as light-emitting diodes, solar cells, photo-detectors, lasers, and biomarkers due to their outstanding electrical and optical performance governed by the quantum confinement effect.
For example, in the application field of light-emitting diodes, since the optical properties of the quantum dots highly depend on the compositions and sizes thereof, it is an important research direction to control the compositions and the sizes of the quantum dots so as to tune the optical properties of the quantum dots.
U.S. Pat. No. 6,322,901 B1 discloses a coated nanocrystalline material of high luminescence and color selectivity, which includes a substantially monodisperse nanocrystalline core and an over-coating uniformly deposited thereon. The particle size of the nanocrystallite core is in a range of about 25 Å to about 125 Å.
U.S. Pat. No. 6,501,091 B1 discloses an electronic device which comprises a population of quantum dots embedded in a host matrix and a primary light source causing the quantum dots to emit secondary light of a selected color. The quantum dots comprise at least one material selected from the group consisting of CdS, CdSe, CdTe, ZnS, and ZnSe. The host matrix comprises at least one material selected from the group consisting of polymers, silica glasses, and silica gels. The quantum dots may optionally be overcoated to increase photoluminescence.
U.S. Pat. No. 6,821,337 B2 discloses a method of synthesizing a nanocrystallite which comprises combining a metal-containing non-organometallic compound, a coordinating solvent, and a chalcogen source to form a nanocrystallite which can be a member of a population of nanocrystallites having a narrow size distribution.
US 2016/0333267 A1 discloses a quantum dot nanocrystal structure, which includes: a core of a compound M1A1, wherein M1 is a metal selected from Zn, Sn, Ph, Cd, In, Ga, Ge, Mn, Co, Fe, Al, Mg, Ca, Sr, Ba, Ni, Ag, Ti and Cu, and Al is an element selected from Se, S, Te, P, As, N, I, and O; an inner shell having a composition containing a compound M1xM21-xA1yA21-y, wherein M2 is a metal selected from Zn, Sn, Pb, Cd, In, Ga, Ge, Mn, Co, Fe, Al, Mg, Ca, Sr, Ba, Ni, Ag, Ti and Cu, A2 is an element selected from Se, S, Te, P, As, N, I and O; and a multi-pod-structured outer shell of a compound M1A2 or M2A2 enclosing the inner shell and having a base portion and protrusion portions extending from the base portion.
US 2016/0369975 A1 discloses a quantum dot-containing wavelength converter, which includes a matrix layer and quantum dots dispersed in the matrix layer. Each of the quantum clots includes a core of a compound M1A1, an inner shell, and a multi-pod-structured outer shell of a compound M1A2 or M2A2. Each of M1 and M2 is a metal selected from Zn, Sn, Pb, Cd, In, Ga, Ge, Mn, Co, Fe, Al, Mg, Ca, Sr, Ba, Ni, Ag, Ti and Cu, and each of A1 and A2 is an element selected from Se, S, Te, P, As, N, I, and O. The inner shell contains a compound M1xM21-xA1yA21-y, wherein M2 is different from M1 and A2 is different from A1. The multi-pod-structured outer shell has a base portion and protrusion portions that extend from the base portion in a direction away from the inner shell.
An approach to directly prepare quantum dots with a controllable multimodal size distribution in a simple one-pot synthesis is presented in an article entitled “Direct Synthesis of Quantum Dots with Controllable Multimodal Size Distribution” by Hsueh-Shin Chen et al. in J. Phys. Chem. C 2009, 113, 12236-12242. Although the quantum dots having different emission wavelengths may be prepared by the approach, the quantum dots are not formed as a core-shell configuration and the quantum yield thereof is relatively low.
A one-pot synthesis of core/shell quantum dots with bimodal size distribution is described in an article entitled “Wide Gamut White Light Emitting Diodes Using Quantum Dot-Silicone Film Protected by an Atomic Layer Deposited TiO2 Barrier” by Guan-Hong Chen et al. in Chem. Commun., 2015, 51, 14750. Bimodal CdSe cores are first synthesized, followed by growing a ZnS shell. Briefly, Se is mixed with trioctylphosphine (TOP) in toluene to prepare TOPSe. CdSe cores are synthesized by injecting TOPSe into a CdO-containing mixture. After a first quantum dot size group is grown, the growth is suspended by quenching, followed by sequential TOPSe injection to grow a second quantum dot size group. Once the bimodal quantum dot cores are grown, a half of ZnS monomers is injected into the reaction mixture at 180° C. After reacting for 10 minutes, the temperature is increased to 210° C., followed by injecting the rest of the ZnS monomers and further growing.
US 2016/0233378 A1 discloses a quantum dot for emitting light and a method for synthesizing the quantum dot. The quantum dot is synthesized in a one-pot method by controlling the rate and extent of a reaction by controlling the following parameters: (i) type and quantity of reactant, (ii) reaction time, and (iii) reaction temperature. Although the emission wavelength of the quantum dot may be adjusted by controlling the aforesaid parameters, the quantum dots with a bimodal size distribution may not be synthesized simultaneously in the one-pot method.